Aluminum hollow materials such as hollow shape materials of square in cross section utilizable for constructive materials and pipes in circle in cross section utilized for a coolant are manufactured from the past according to the port hole extrusion method.
The port hole extrusion method is employed for the manufacture of hollow materials and the like of a relatively soft aluminum alloy, such as JIS 1000 series (pure Al series), 3000 series (Al—Mn series), 6000 series (Al—Mg—Si series), and 7000 series (Al—Zn—Mg series) free from copper.
In the aforesaid port hole extrusion method, an extrusion billet is manufactured by casting a given aluminum alloy according to an ordinary DC casting method (a semi-continuous longitudinal casting method) or a hot top casting method to form an ingot, subjecting the ingot to a homogenizing treatment to reduce segregation, and cutting off the ingot to pieces having a given length.
The extrusion billet is thereafter re-heated by a low frequency induction furnace (induction heater) or a gas-heating furnace and hot extruded to a hollow material. The re-heating temperature is determined by considering extrusion property or quality of the extrusion material within a temperature range of 370˜530° C., and 400˜500° C. in majority of the cases.
The aforesaid hollow materials manufactured by way of the port hole extrusion may be subjected to a draw-elongation processing for accuracy of dimensions and for reducing the diameter. The drawing-elongation processing includes a method wherein a short hollow material is pulled out by the aid of a draw bench and a method wherein a long hollow material is pulled out by a continuous drawing-elongation machine with the aid of a floating plug. The hollow material after drawing-elongation processing is subjected to a solid solution treatment, an aging treatment, an annealing treatment, etc. for imparting strength and workability thereto according to the intended use.
On the other hand, aluminum alloy pipes are used from the past for heat exchanger pipes of automobiles for the purpose of lightweight.
For example, JIS 6063 alloy (a representative composition: Al—0.5 wt % Mg—0.35 wt % Si) or JIS 3003 alloy (a representative composition: Al—1.0 wt % Mn—0.1 wt % Cu—0.1 wt % Si—0.4 wt % Fe) having corrosion-resistance to external environment and strength tolerant to coolant pressure and vibration of engine or compressor is widely used as coolant pipes for automobile coolers.
The above-mentioned JIS 6063 alloy is used as pipes especially requiring vibration-resisting fatigue strength, the so-called flex hose while the above-mentioned JIS 3003 alloy is widely adopted as a metallic piping portion for automobile coolers, etc.
Pipes comprising JIS 3003 alloy are circular pipes having an outer diameter of about 6˜19 mm and a wall-thickness of about 0.8˜1.2 mm and their production is carried out, for example, according to the following steps:
First of all, JIS 3003 alloy is cast into a round bar ingot according to a DC semi-continuous longitudinal casting method. The round bar ingot is then subjected to a homogenizing treatment where the ingot is heated at a high temperature to avoid segregation of alloy components and impurities. After cutting off the ingot into pieces of a given length, extruded billets are formed, which are re-heated and subjected to a mandrel extrusion method where extruded pipes are extruded. The extruded pipes are subjected to a drawing-elongation processing by which pipe materials of a desired shape are pulled out and are further subjected to annealing for removing processing strain and furnishing them with a proper workability.
In a conventional manufacturing process, a large size billet having an outer diameter of at least 14 inches is extruded according to a mandrel extrusion method to form a extruded pipe of a large diameter and a thickened wall, which is treated by a continuous drawing-elongation machine to provide multipass drawing-elongation processing.
In passing, the aforesaid homogenizing treatment of ingot has influence on the quality of the final product so that the conditions therefor are determined by considering economic factors such as alloy components, easiness in extrusion processing, required characteristics of product, energy cost for the homogenizing treatment, time, etc.
The conditions (maintained temperature and maintained time) for the homogenizing treatment of a practical aluminum alloy subjected to hot extrusion are briefly as follows:
JIS 1050 alloy: 520˜560° C., 4˜10 hrsJIS 1100 alloy: 520˜560° C., 4˜10 hrs,JIS 3003 alloy: 570˜610° C., 4˜10 hrs,JIS 3004 alloy: 530˜580° C., 4˜10 hrs,JIS 6063 alloy: 520˜580° C., 4˜10 hrs, andJIS 7NO1 alloy: 450˜490° C., 4˜10 hrs.
In this case, cooling of the alloy from the maintained temperature to ordinary temperature is carried out by air-cooling with a fan, leaving to stand, sprinkling water by the aid of a sprinkler, etc.
The pipe materials thus manufactured are further subjected to a terminal end processing and a bending processing for the use as cooler pipes for automobiles, etc.
In the above-mentioned terminal end processing of pipe materials, a pipe expanding processing, pipe condensing processing, and forming by rolling are combined to form a variety of beads (joint portion). As a high reliability is required for the beads, a new processing method called shaft seal bead is widely adopted. This shaft seal bead is complicate in shape so that a higher workability is required for pipe materials.
Further, pipes for air conditioner of automobiles require good brazing property and high quality, which is maintained even by heating for brazing.
Aluminum pipe materials for cooler piping still further require strength tolerant to vibration and plastic workability, and a moderate balance of strength and ductility is desired.
For example, the mechanical characteristics of pipes manufactured from JIS 3003 alloy according to mandrel extrusion-drawing elongation-annealing steps are a tensile strength of 95-125 N/mm2, 0.2% yielding strength of at least 35 N/mm2, and an elongation of at least 30%.
Besides, an external pipe surface of the pipes for automobiles requires anti-corrosive property and formability.
The formability requires finely crystalline grains incapable of causing surface roughening at the time of processing, dimensional accuracy such as an outer diameter, a wall-thickness, etc., brazing property, and the like.
With respect to a process for manufacturing aluminum alloy pipes for automobile piping, a process for simplifying steps is now under discussion for reducing cost, wherein the mandrel extrusion is changed to a port hole type continuous extrusion and port hole extrusion pipe materials are directly used as piping materials for a heat exchanger to omit the drawing-elongation processing and annealing step.
Meanwhile, the port hole extrusion method is an extrusion method wherein extrusion raw materials are divided into cleaved bodies with plural port holes and these bodies are welded to an integrated body at the exit of the port holes, thus forming plural welding portions are formed in the longitudinal direction at a given position on the cross section of the extruded materials.
Namely, in case of extrusion with four ports, an aluminum alloy is once divided into 4 cleaved bodies and the cleaved bodies are integrally welded in a welding chamber at an extrusion die and concurrently passed through a clearance between the die bearing portion and the mandrel to form an extruded material of a desired shape so that plural continuous welding portions exist in the lengthwise direction and remain even after the drawing-elongation processing.
When pipe materials of JIS 3003 alloy manufactured according to the port hole extrusion method are exposed to a corrosive environment, however, there arises a problem that the welding portions undergo significant corrosion (referred to hereinafter as the preferential corrosion in the welding portion).
For example, the preferential corrosion of the pipe material obtained according to the port hole extrusion method with 4 port holes is such that as in FIG. 1, a continuous corrosion in the longitudinal direction is preferentially generated in the 4 welding portions, besides corrosions at non-welding portions.
This preferential corrosion in the welding portion is extremely rapid in corrosion velocity so that a penetration hole is formed within a short period of time. In case of subjecting pipes for air conditioners of automobiles having a wall-thickness of 1 mm to the CASS test, for example, pitting corrosion in a non-welding portion is not penetrated even after the lapse of 400 hrs but pitting corrosion in the welding portion is penetrated within 200 hrs.
Concerning this preferential corrosion, the front end portion side of extrusion (the former half portion) tends to become more corrosive than the rear end side portion (the latter half portion). It has become an important technical subject, therefore, that anti-corrosive property is maintained in the welding portion over the full length of the extruded material in the lengthwise direction.
This preferential corrosion of the welding portion easily tends to take place in an Al—Mn series alloy and is generated in case of the Mn content being at least 0.3 wt % and rapidly proceeds in case of the Mn content exceeding 0.8 wt %. By the way, Mn serves to increase deformation resistance while reducing extruding property so that the upper limit of the Mn content in case of the port hole extrusion is around 1.5 wt %.
As the Al—Mn series alloy excels in strength and anti-corrosive property, JIS 3003 (Mn amount 1.0˜1.5 wt %), JIS 3203 (Mn amount 1.0˜1.5 wt %), and JIS 7N01 (Mn amount 0.2˜0.7 wt %) alloys are widely employed. Since hollow materials according to the port hole extrusion involves a problem of the aforesaid preferential corrosion, however, the application of these materials has often refrained from the use where anti-corrosive property is regarded important.
In addition to this, a variety of obstacles such as bad appearance in color tone or luster, etc. takes pace between the welding portion and non-welding portion.
In conventional aluminum alloy pipe materials for air conditioner piping of automobiles, there exist a number of fine striation (confirmed definitely as grooves by enlarged observation) in lengthwise direction formed by the drawing-elongation processing. This striation defect gives damage on sealing property and causes injury of O-ring rubber for sealing. Thus, a drastic solution of this defect is required.
An object of the present invention is to provide a hollow material of an Al—Mn series alloy manufactured according to the port hole extrusion process and improved in preferential corrosion in welding portions. Another object of the present invention is to provide a process for the manufacture of the aforesaid hollow material of an Al—Mn series alloy.
Further, an object of the present invention is to provide extruded pipe materials of aluminum alloys for air conditioner piping, which are improved in anti-corrosive property and fine striation on the surface. Besides, an object of the present invention is to provide a process for manufacturing the aforesaid extruded pipe materials of aluminum alloys for air conditioner piping at a low cost.
The above-mentioned and other objects, features and advantages of the present invention will become more apparent from the following descriptions.